1. Field of the Invention
The present invention relates to a lens driving device, more particularly, capable of adjusting an effective focal length determined by the relative distance of lenses in an optical device having a zooming function.
2. Description of the Related Art
Various optical instruments such as a camera, a camcorder, a zoom camera, an observation camera and an optics in a Micro Air Vehicle (MAV) have a driving structure for enabling forward and backward movement of a lens for zooming. There have been developed several structures for transporting the lens for such zooming.
A conventional driving technique based upon a cam structure has been used for zooming to adjust a focal length. The cam structure-based driving technique executes zooming by varying the relative spacing between lenses along a lens barrel, which is driven by an electromagnetic motor, and a cam-shaped groove placed in a lateral portion of the lens barrel.
FIG. 1 illustrates a conventional lens driving mechanism which is disclosed in U.S. Pat. Ser. No. 6, 268, 970, entitled “Zoom Lens Barrel.” In FIG. 1, lens groups 120, 130 and 140 are supported by frames which are in turn supported by cam tubes 160 and 170. The cam tubes are driven by a plunger 110 to shift a lens axially.
The cam structure-based zooming technique determines the relative position of each lens according to the configuration of a cam. Therefore, there are drawbacks in that this technique further requires a driving unit and a focusing lens for setting a focus at a specific magnification as well as complicates a driving mechanism including a driven reduction gear and a lens holding structure which moves along the cam.
FIG. 2 illustrates a conventional zoom lens mechanism of a camera which is disclosed in Korean Laid-Open Patent Publication Serial No. 2000-55180. The zoom lens mechanism comprises a camera body 200 and a stationary lens group 201 including a plurality of lenses. Within the camera body 200, there is provided a space for receiving a zoom motor 203. The zoom motor 203 has a shaft coupled with a lead screw 205 which has threads and grooves formed in the outer periphery thereof. A clip 207 for transmitting power is coupled with the outer periphery of the lead screw 205. The clip 207 is also provided with threads and grooves in a portion thereof contacted with the lead screw 205, in which the threads and grooves are shaped equal to the those of the lead screw 205 to mesh with the same. The clip 207 is also coupled with a zoom barrel 209 which is in turn coupled with a movable lens group 202. The zoom barrel 209 is coupled slidably with a guide shaft 211 which is oriented along an optical axis so that the zoom barrel 209 can be moved in a direction of the optical axis along with the guide shaft 211.
In the zoom lens mechanism of the above camera, rotation of the motor 203 also causes the lead screw 250 to rotate along with the motor 203. Then, rotation of the lead screw 205 is translated into linear motion via the clip 207 so that the clip 207 moves linearly along the optical axis. As the clip 207 performs linearmovement, the zoom barrel 209 in turn moves along the optical axis. When the zoom barrel 209 moves along the optical axis, a portion of the zoom barrel 209 in contact with the guide shaft 211 performs sliding motion so that the zoom barrel 209 can perform forward and backward movement along the optical axis in a predetermined range.
However, the conventional zoom lens mechanism has a problem of electromagnetic wave since the motor is driven by electromagnetic power. As a result, this mechanism is rarely applicable to small-sized communication devices. Further, the electromagnetic motor uses a final reduction gear thereby making a mechanical structure complicated. Moreover, the zoom lens and the focus lens must be moved separately in order to be focused.
Recently, there is developed a micro-optical zoom mechanism in order to overcome the above drawbacks as well as to impart a zooming function to a micro optical instrument. According to the current trend, the micro optical instrument employs an intelligent device such as a piezoelectric element rather than the conventional driving technique using the electro magnetic motor. Substituting the piezoelectric element for the conventional motor driving technique has advantages that a driving structure can be simplified and high efficiency can be realized owing to a direct drive mechanism.
FIG. 3 shows an example of a zoom lens driving device using such a piezoelectric element which is disclosed in U.S. Pat. No. 6,215,605, entitled “Driving Device.” The lens driving device in FIG. 3 has piezoelectric actuators 311 and 312 fixed to base blocks 321 and 322, and transfers displacement to driving rods 316 and 317 so as to transport lenses L2 and L4 under effects of the preload from projections 331a and 332a together with inertia and acceleration of lens holders 331 and 332. The piezoelectric actuator 312, according to the waveform of exciting input, transports the lens by enabling the lens holder to move together with the driving rod or to slide and stay in position. The piezoelectric actuator 312 can also transport the lens in forward and backward directions.
The lens driving device shown in FIG. 3 is arranged in use as shown in FIG. 4, in which the piezoelectric actuators 311a and 311b are arranged adjacent to each other. So, when a base block 313 receives any expansion/compression transferred from one of the piezoelectric actuators 311a and 311b, this expansion/compression may be also transferred to the other one of the piezoelectric actuators and thus its corresponding lens. Therefore, the base block dl3 is provided with a groove dl3g in order to block transfer of the expansion/compression between the piezoelectric actuators. However, the groove complicates the structure of the driving device while causing fabrication of the driving device difficult. Furthermore, the groove cannot block the expansion/compression interference between the piezoelectric actuators completely.
Further, the length of the driving rods 316 and 316, which are reciprocated by the piezoelectric actuators to transport the lenses, is largely restricted according to the size of the piezoelectric actuators. Such restriction to the length of the driving rods acts as limitation to the transportable distance of the lens and thus disadvantageously affects qualities of a product incorporating the lens driving device.
Because the driving rods are essentially fixed, the conventional lens driving device has following problems: It is impossible to vary the length of the lens barrel on which the lenses are internally mounted. In addition to a space used for transport of the lens, an additional space for arranging driving elements is required. As a result, the overall size of the driving device is rarely reduced. Furthermore, the lens is partially supported by the driving rod so that asymmetric expansion/contraction may occur in the lens during actuation, thereby potentially making the lens driving operation unstable.